Stepper motors are used in a wide variety of applications that require precise motion control, such as in printers, scanners, x-y tables, turntables, tape and disk drive systems, security cameras and other optical equipment, robotics, CNC (computer-numeric-control) machine tools, dispensers, and injector pumps. Unlike “conventional” AC or DC motors, which produce continuous rotary motion from a continuously applied input voltage, stepper motors will stay indefinitely at a particular stable “detent” position as long as the electrical power is maintained. An electrical phase change, applying power to a different set of stator coils, is required to make the motor rotate to a new stable detent position. A stepper motor's movement is made up of a series of discrete incremental rotational steps.
One goal in stepper motor design is to reduce noise and vibration caused by erratic jerking motion of the discrete steps between the successive stable detent positions. Another goal is to increase both resolution (number of steps per revolution) and accuracy of the motor positions. Other goals are to provide adequate holding torque and efficient power usage over a range of motor speeds. These various design goals are met in a variety of ways, often involving tradeoffs. For example, mechanical damping has been used to smooth out the motion, but it also adds load to the motor and cannot improve step accuracy. In U.S. Pat. No. 6,008,561 to Tang, a motor is provided with auxiliary damping windings that are coupled to form a closed current loop. The damping windings absorb energy from or provide energy to the phase windings by mutual induction. The effects of such electromagnetic damping are similar to that of mechanical damping.
Different modes of driving a steeper motor can affect both the positional resolution and smoothness of motion. For example, a microstepping mode of operation allows a full step to be divided into as many as 500 micro-steps, which provides a potential resolution on the order of 100,000 micro-steps per revolution, assuming 200 full steps per revolution in a 1.8 degree stepper motor. This microstepping is achieved by limiting the drive current that the controller sends to the groups of motor coils at each step so that the current waveforms are approximately sinusoidal instead of simply 100% on/off. The unequal pull of partially energized coils causes the rotor to assume intermediate positions between the full-step positions. Microstepping can improve the smoothness of motion for quieter operation in comparison to full-step and half-step drive modes, with some loss of torque as a tradeoff.
However, unlike the full-step positions, the micro-steps are not guaranteed to be equal in size. Because of detent torque, coil inductance, pole geometry, and other factors, even if the drive current waveform applied to the stator coils should happen to be perfectly sinusoidal in form, a perfectly linear response by the motor will generally not be achieved. In conventional designs, rotor and stator teeth are aligned where full current is applied at one-phase ON stable positions (i.e., one phase has 100% current applied to a set of stator coils, while the other phase is at a zero crossing point with 0% current applied to another set of stator coils). The rotor has greater difficultly pulling out from these stable positions, which typically results in erratic jerks in rotor motion. RMS Technologies, headquartered in Carson City, Nev., has developed its R325 drive to output a predetermined amount of optimal holding and running current into the motor to overcome the motor's detent torque and thereby substantially reduce the jerk at the stable zero crossing points for greater linearity and accuracy of motion.
The present inventor has also contributed to the advancement in stepper motors, as exemplified in prior U.S. Pat. Nos. 4,638,195; 4,910,475; 6,114,782; 6,597,077; and U.S. Pat. No. 6,969,930. In one of the aforementioned patents ('077), bifilar windings around the stator poles are connected to a driver in a manner (T-connection) different from the conventional series and parallel stator coil connections, that in addition to maximizing torque at medium speeds also smoothes stepping motion and reduces vibrations compared to the conventional connections. In another of the aforementioned patents ('930), the bifilar winding ratio is chosen (1: tan x) in order to shift the torque profile by an angle x in a half-stepping motor so that peak torque no longer coincides with one-phase ON or two-phase ON positions, resulting in smoother motion.
Other techniques have been devised to reduce torque variability in stepper motors, such as by modifying the relative dimensions or displacing the positions of one or more groups of poles in order to break rotational symmetries in motor geometry. For example, U.S. Pat. No. 5,852,334 and U.S. Pat. No. 6,060,809 to Pengov employ a rotor with alternately wide and narrow pole faces. U.S. Pat. No. 4,739,201 to Brigham et al. shows how one can reduce any harmonic of the torque/angle curve in a hydride stepper motor by displacing a first set of pole teeth from their “normal” one-half tooth pitch position relative to a second set of pole teeth by a displacement angle calculated to cancel the harmonic generated by one set with that generated by the other set. Providing a motor with different numbers of rotor and stator poles and/or teeth with correspondingly different pitches and rotational offsets with respect to each other can effectively average the magnetic field's influence on torque, as described in U.S. Pat. Nos. 4,423,343; 4,647,802; 4,675,564; 5,157,298; and U.S. Pat. No. 5,309,051. For example, in the last named ('501) patent to Kobori, the stator teeth pitch is selected relative to the rotor teeth pitch and the number of salient poles in order that the stator teeth on different poles differ in their phase relation to the rotor teeth.
While all of these various approaches achieve some degree of smoothing of stepper motion and noise reduction, the smoothing is typically accompanied by some reduction in microstepping accuracy or in holding torque, or other performance factor, as a tradeoff. It is desirable that any such tradeoffs are minimized or eliminated altogether to the extent possible.